In well logging techniques, a metal pipe known as "casing" is placed inside a well for the purpose of consolidating it, and cement is injected in the annular gap between the casing and the surrounding ground. Perforations are then made through the casing and the cement at a level where the earth formations are expected to produce hydrocarbons, so as to allow the hydrocarbons to flow towards the surface up a production tubing previously disposed coaxially inside the casing.
It is most important to have knowledge of zones in which fluid enters the well and zones in which it leaves the well, and to ensure that the annular cement-filled space between the casing and the ground is fluidtight. However, underground formations contain fluids other than hydrocarbons, in particular water and sometimes gas. Owners and/or operators of hydrocarbon wells are interested in producing hydrocarbons and possibly gas, with the production of water constituting a handicap and an operating loss. Unfortunately, it happens relatively frequently that undesirable flows of water appear in the cement in a longitudinal direction parallel to the well between the casing and the ground. This phenomenon has the undesirable effect of communicating with one another underground formations that are located at different depths, i.e. that are at different pressures, thereby enabling water from a first formation to mix with hydrocarbons from another formation situated at a different depth along the well.
The invention may also be implemented in production wells, and more particularly in so-called "injection" wells, which term covers various families of wells. In a first family, fluid is injected at high pressure into a first well in order to cause hydrocarbons in the underground formations to move towards a second well adjacent to the first, thereby causing it to become a production well. A second family of injection wells is intended to receive fluids for storage purposes, either fluids that result from hydrocarbon production such as brine, or else liquid wastes. In both of these families of injection wells, it is important to ensure that such a well is fluidtight in order to prevent the fluids that are injected contaminating under ground sources of drinking water (the water table). Such contamination is generally due to cracks or passages through the annular portion filled with cement and/or in the ground in the proximity of the wall of the well. To this end, injection wells are generally subjected to surveys prior to injection and also to periodic inspections for verification purposes.
The invention may also be applied to determining the quantity of water flowing in a pipe that conveys a mixture of water and of hydro carbons. This applies, for example, to a pipe running from a hydro carbon-producing well and conveying the hydrocarbons (possibly mixed with water) to a storage tank. The operator then needs to know whether the fluid leaving the well is constituted entirely of hydrocarbons, and if not, the operator needs to know how much water is mixed in with the hydrocarbons.
It will thus be understood that the method of the invention is advantageously applicable to determining information relating to a flow of water when the flow takes place under physical conditions that make direct measurement impossible, e.g. in a hydrocarbon well or in a pipe conveying a mixture of water and hydrocarbons in unknown proportions.
Of the various methods that have already been proposed for detecting and determining the characteristics of a flow of water in a well, whether inside casing or between the casing and the ground, the nuclear method based on oxygen activation seems to be promising. This method has been known, in its application to oil well logging at least since 1967, as can be seen from the article "Advances in nuclear production logging" by P. A. Wichmann et al., Transactions SPWLA (1967). The method is implemented by means of a tool or sonde including a nuclear source suitable for emitting high energy neutrons, and a detector for detecting gamma radiation. The neutrons emitted by the source are at an energy such that they interact with the atoms of oxygen in the water so as to "activate" these atoms, i.e. transmute them into the form of unstable nitrogen N.sup.16. Atoms of N.sup.16 nitrogen return to a stable state in the form O.sup.16 oxygen in compliance with an exponential time relationship having a half-life of 7.13 seconds, and in so doing they emit gamma radiation at 6.13 MeV and at 7.12 MeV. This activation reaction is also known as the O.sup.16 (n,p)N.sup.16 reaction. The count rate in the gamma ray detector (corresponding to the number of gamma particles detected) is proportional to the total quantity of oxygen present around the sonde, and thus to the water flow rate.
Various attempts have been made at implementing methods and providing operational probes capable of being used in hydrocarbon wells. Most of these methods rely on the ratio between the number of photons detected by two different gamma radiation detectors that are longitudinally spaced apart relative to the neutron source.
However, those known methods relying on two detectors suffer from drawbacks.
Downhole measurements require very long measuring time, up to several minutes. Unfortunately, time is an extremely important factor when operating hydrocarbon wells, for financial reasons. In addition, such measurements are capable of providing only the average velocity of the water flow and not its mass flow rate. In addition, these methods require preliminary calibration, even when merely determining the velocity of the water flow, in other words, the response of the tool to given external conditions is measured, i.e. to different flow velocities, flow rates, and radial flow distances, in order to obtain a set of reference signals; these reference measurements which are assumed to correspond to the "stationary" oxygen as opposed to the moving oxygen representative of the flow itself are then subtracted from the real measurements; calibration may be performed either using laboratory equipment specially constructed for this purpose or else in a portion of the well that is assumed to be free from any flow of water, but which nevertheless has characteristics that are identical to the characteristics of the portions of the well being inspected. However, the calibration operation is lengthy and therefore expensive. In addition, if performed in the well, it is of relatively low reliability since there is no way of ensuring that the region selected as a reference is indeed representative and free from any flow of water. Finally, known methods suffer from limitations concerning the range of flow velocities that can be detected accurately, in particular when velocities are relatively low, e.g. below 0.016 meters per second (3 feet per minute). This results in part from limitations due to the above-described calibration. This disadvantage is all the greater as the water flow velocities generally encountered in wells are often low, i.e. about or less than 0.016 meters per second.
Furthermore, the oxygen activation method raises another difficulty. The emitted neutrons interact with, and therefore activate, any oxygen atoms present around the tool, i.e. both "moving oxygen" and "stationary oxygen". Stationary oxygen is to be found mainly in the underground formation and in the cement. The moving oxygen is the oxygen present in the fluids flowing downwards or upwards along the well. It is therefore important to separate the stationary oxygen from the moving oxygen since it is only the moving oxygen that is of interest since it represents the flow of water.
Variants of methods relying on oxygen activation use a single detector and rely on adapting the displacement speed of the sonde as a function of the flow speed of the water, as described, for example, in the article entitled "Examples of detection of water flow by oxygen activation on pulsed neutrons logs" by W. H. M. DeRosset, in the CCC heading of SPWLA 27 Annual Logging Symposium of 9-13 June 1986. However, that method relies on preliminary or estimated knowledge of the flow speed of the water, and it is also sensitive to the spacing between the source and the detector.
European patent application No. 0 421 844 entitled "Nuclear oxygen activation method and apparatus for detecting and quantifying water flow", discloses a method and a sonde based on oxygen activation and seeking to enable water flow velocity to be determined. In the method described in that European patent application, a relationship representative of the flow (moving oxygen) is extracted from a (preferably graphic) representation of the number of gamma rays detected (or of the count rate) as a function of time. A characteristic of that relationship leads to the flow velocity of water in simple manner. Provision is also made for subtracting from the measurement, those gamma radiation counts that constitute "background noise" and that are constituted mainly by gamma rays produced by the radioactive elements present in the cement and in the underground formation. The graphic representation takes different shapes as a function of the duration of neutron emission (i.e. the irradiation duration), and as a function of the distance between the detector and the neutron source. Nevertheless, the portion of the curve representative of the flow is identifiable in the overall curve and may be deduced therefrom.
The method proposed in that European patent application gives satisfaction. However, although it is useful to know the velocity of the water flow, the information which is generally most required is the quantity of water that is flowing; in this respect the preferred parameter is flow rate. The method described in the above-mentioned European patent application provides for the mass flow rate of the flow being determined by means of a formula based on the velocity of the flow as measured by the above-mentioned method. Although constituting useful additional information about the flow, the method proposed for determining the flow rate suffers from certain limits. In particular, the flow rate is based, amongst other parameters, on velocity which is, in fact, a mean velocity that may merely be an approximation, in certain circumstances. In addition, the velocity measurement has relatively low accuracy, both for high velocities (greater than 50 m/s) and for low velocities (less than 0.01 m/s). Alternatively, that European patent application proposes another method for determining the mass flow rate, based on preliminary establishment of a reference curve for gamma radiation count rate as a function of flow rate. This reference curve is established before performing the measurement, e.g. in a laboratory installation. Thereafter, during real measurement, the total number of gamma rays representative of the flow is determined from the characteristic curve of the flow (gamma number as a function of time). The looked-for mass flow rate is the rate on the reference curve that corresponds to the real count number. This alternative method assumes that a reference curve has been established, thereby complicating the procedure and increasing costs.
In conclusion, the method and its variants proposed in the above- mentioned European patent application, although satisfactory overall, is nevertheless capable of being improved.